This invention pertains to a method for preparing electrodes and to their use in electrolytic cells, for example, brine electrolysis cells.
There are three general types of electrolytic cells used for the production of chlor-alkali: (1) the mercury cell, (2) the diaphragm cell, and (3) the membrane cell. The operation of each of these cells is discussed in Volume 1 of the Third Edition of the KIRK-OTHMER ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY, page 799 et. seq. Other electrolytic cells which employ electrodes for electrolysis of aqueous solutions are the so-called "chlorate cells" which do not use a divider or separator between the cathodes and anodes. In the mercury cell, the alkali metal values produced by electrolyzing an alkali metal salt form an amalgam with the mercury; the amalgam, when reacted with water, produces NaOH and frees the mercury which can be recovered and cycled back for further use as a liquid cathode.
In many chlor-alkali electrolytic processes a brine solution (electrolyte) is electrolyzed by passing electric current therethrough in a cell having a diaphragm separator means or a membrane separator means positioned between the cathode means and the anode means. Chlorine is produced at the anode means while sodium hydroxide (NaOH) and hydrogen (H.sub.2) are formed at the cathode means. Brine is fed continuously to the cells, while Cl.sub.2, NaOH and H.sub.2 are continuously withdrawn from the cells.
The minimum voltage required to electrolyze an electrolyte into Cl.sub.2, NaOH and H.sub.2 may be calculated using the thermodynamic data. However, in commercial practice, the theoretical amount of voltage is not achievable and higher voltages must be used to overcome the various resistances inherent in the various types of cells. To increase the efficiency of the operation of a diaphragm or a membrane cell one may attempt to reduce the overvoltages of the electrodes, to reduce the electrical resistance of the diaphragm or membrane, or reduce the electrical resistance of the brine being electrolyzed. The invention herein described results in an electrode particularly useful as a cathode in the electrolysis of brine; cathode overvoltage is substantially reduced, resulting in increased power efficiencies.
Because of the multi-million-ton quantity of alkali metal halides and water electrolyzed each year, even a reduction of as little as 0.05 volts in working voltage translates to very meaningful energy savings. Consequently, the industry has sought means to reduce the voltage requirement.
Throughout the development of chlor-alkali technology, various methods have been developed to reduce the cell voltage. Some practitioners have concentrated on reducing cell voltage by modifying the physical design of the electrolytic cell, while others have concentrated their efforts on reducing the overvoltage at the anode or the cathode. The present disclosure pertains, in part, to a novel process to make an electrode that is characterized by a significantly low overvoltage and to the use of these electrodes in electrolytic cells.
It has been disclosed that an electrode's overvoltage is a function of the current density and its composition (reference: PHYSICAL CHEMISTRY, 3rd ed., W. J. Moore, Prentice Hall (1962), pp. 406-408), where the current density refers to the amperage applied per unit of true surface area of an electrode and composition refers to the chemical and physical makeup of the electrode. Therefore, a process that will increase an electrode's surface area should decrease its overvoltage at a given apparent current density. It is also desirable to use a composition of matter that is a good electrocatalyst; this further reduces the overvoltage.
It is well known in the art to use plasma or flame spraying to coat an electrode with an electroconductive metal. In U.S. Pat. No. 1,263,959 it was taught that anodes may be coated by spraying fine nickel particles onto an anode, wherein the particles are rendered molten and impacted on the iron substrate by means of a blast.
Cathodes, also, have been coated with electroconductive metals. In U.S. Pat. No. 3,992,278, cathodes were coated by plasma spraying or flame spraying an admixture of particulate cobalt and particulate zirconia. When these electrodes are used for the electrolysis of water or an aqueous alkali metal halide salt solution, they are said to give prolonged lowering of hydrogen overvoltage.
Various metals and combinations of metals have been used to coat electrodes by plasma or flame spraying: U.S. Pat. No. 3,630,770 teaches the use of lanthanum boride; U.S. Pat. No. 3,649,355 teaches the use of tungsten or tungsten alloy; U.S. Pat. No. 3,788,968 teaches the use of titanium carbide or titanium nitride and at least one metal and/or metal oxide of the platinum group and a second oxide coating which is porous; U.S. Pat. No. 3,945,907 teaches the use of rhenium; and U.S. Pat. No. 3,974,058 teaches the use of cobalt as a coating with an overcoat of ruthenium.
It is, likewise, well known in the art to make porous electrode coatings by selective leaching. Coating an electrode with particulate nickel, then sintering the nickel as taught in U.S. Pat. Nos. 2,928,783 and 2,969,315; electrodepositing an alloy onto a substrate then leaching out one component of the alloy as taught in U.S. Pat. No. 3,272,788; pressing or cementing two or more components together or onto an electrode substrate and then selectively leaching out one or more of the coating components as illustrated by U.S. Pat. Nos. 3,316,159; 3,326,725; 3,427,204; 3,713,891 and 3,802,878.
It is also disclosed in the art to combine the steps of making electrodes by plasma- or flame-spraying followed by leaching. It is also disclosed to combine the steps of electroplating followed by leaching. Examples of known methods are illustrated in the following patents; U.S. Pat. No. 3,219,730 teaches coating a substrate with a multiple oxide film coating then removing the substrate by leaching, thus forming an electrode; U.S. Pat. No. 3,403,057 teaches flame or plasma spraying a Raney alloy onto a substrate followed by leaching aluminum out of the alloy thus leaving a porous electrode; U.S. Pat. No. 3,492,720 teaches plasma spraying tungsten, titanium or alloys thereof along with aluminum, thorium and zirconium oxides onto a substrate. The substrate was subsequently removed, leaving a porous electrode.
U.S. Pat. No. 3,497,425 teaches preparing porous electrodes by coating the substrate with a relatively insoluble metal followed by a coating of a more easily dissolvable metal. The teaching requires heat treating to cause inter-diffusion of the two coats, while optimum conditions require separate heat treatments for each coat. The dissolvable metal is subsequently leached out, leaving a porous electrode. U.S. Pat. No. 3,618,136 teaches forming porous electrodes by coating a binary salt composition onto a substrate and leaching a soluble component from the system. The patent teaches that it is critical that the binary salt mixture is a eutectic composition and that optimum results are obtained when the same anions are used for both the active and the inactive salts, e.g. silver chloride--sodium chloride.
Netherlands Patent Application No. 75-07550 teaches the preparation of porous cathodes by applying to a substrate a coating of at least one non-noble metal from the group of nickel, cobalt, chromium, manganese and iron, alloyed with a secondary, less noble, sacrificial metal followed by removal of at least a part of this sacrificial metal. Specifically, the sacrificial metal is chosen from the group of zinc, aluminum, magnesium and tin. The sacrificial metal is removed by leaching with a lye solution or an acid solution.
Japanese Pat. No. 31-6611 teaches forming a porous electrode by electroplating onto a substrate a nickel coating followed by a coating of zinc or some other soluble substance which is soluble in an alkaline solution. These coated electrodes are then either immersed in an alkaline solution or subjected to an electrochemical anodizing treatment to elute and remove zinc and other soluble substances, thus forming a porous electrode. Prior to immersion, a heat treatment of the coated electrode is required in some embodiments.
U.S. Pat. No. 4,279,709 discloses a method for making electrodes including electrodes having reduced overvoltage by applying an admixture of particulate metal and a particulate inorganic compound pore-former and then leaching out the pore-former to form pores.
Electrodes of film-forming metal substrates, especially titanium, coated with oxides of Group VIII metals of the Periodic Table of The Elements have been taught, especially conjointly with other metal oxides. as being useful as anodes in electrolytic processes, such as in brine electrolysis. Ruthenium oxides, platinum oxides, and other oxides of the "platinum metal series", in association with various other metal oxides have received much acclaim as coatings for valve metal substrates (esp. Ti) for use as anodes. Patents relating to such anodes are, e.g. U.S. Pat. Nos. 3,632,498 and 3,711,385. These coatings may be applied in several ways, for example, U.S. Pat. No. 3,869,312 teaches that platinum group metal oxides, combined with film-forming metal oxides may be deposited on valve metal substrates by applying a mixture of thermally-decomposable compounds of platinum group metals and a thermally-decomposable organo-compound of a film-forming metal in an organic liquid vehicle which may also optionally contain a reducing agent, to a support member, drying the coating by evaporation of the organic vehicle, then heating the member in the range of 400.degree.-550.degree. C. to form metal oxides. Repeated coats are applied to increase the thickness of the coating. Also an overcoating of a film-forming metal oxide is applied. U.S. Pat. No. 3,632,498 teaches that coatings of finely divided oxides of platinum group metals and film-forming metals may be produced by use of a plasma burner, by heating substrates which have been coated with thermally-decomposable compounds of platinum group metals and film-forming metals, by electrically depositing the metals in a galvanic bath followed by heating in air to form the oxide, among others.
Some further patents relating to electrodes having retal oxide surfaces are, e.g., U.S. Pat. Nos. 3,616,445; 4,003,817; 4,072,585; 3,977,958; 4,061,549; 4,073,873; and 4,142,005.
The use of platinum group metal oxides, particularly ruthenium oxide, in acitve coatings for the evolution of hydrogen is also known (ref. Melendres, Carlos A., SPRING MEETING ELECTROCHEM. SOC., May 11-16, 1975). Japanese patent publication No. 9130/65, application (OPI) Nos. 131474/76 and 11178/77 refer to the use of a mixture of platinum group metal oxide(s) with another metal oxide as active cathode coatings. U.S. Pat. No. 4,238,311 teaches that a cathode coating consisting of fine particles of platinum group metals and/or platinum group metal oxides in nickel is useful as a cathode coating.
In general, it is known by those skilled in the art that the use of oxides of platinum group metals as active catalysts for the evolution of hydrogen in modern electrolytic chlor-alkali cells employing permionic membranes is not useful because of extreme conditions of NaOH concentration and temperature now possible, wherein NaOH concentrations of 30% and temperatures exceeding 95.degree. C. are not uncommon. Oxide coatings prepared according to the known art are found to decrepitate with use and fail by loss of adherence to the substrate, accompanied presumably by substantial reduction, in some cases, to base metals.
It is also well known to those practiced in the art that catalytic coatings consisting of metals with intrinsically low hydrogen overvoltage properties are subject in actual practice to loss of catalytic activity due to overplating with metallic contaminants, such as iron for example, which are commonly present in brine and water employed in the process of electrolysis. Consequently, active coatings found useful by those practiced in the art for evolution of hydrogen in modern electrolytic membrane chlor-alkali cells are limited to the type characterized by high surface area, or porous coatings, with compositions resistant to some degree to chemical attack at these conditions, e.g. nickel or various stainless steels.
In these cases, the full effect of the catalytic nature of intrinsically low hydrogen overvoltage catalysts are not realized in practice, since, as is well known to those practiced in the art, the performance of these essentially high surface area coatings degrades in time to a level characterized by the equivalent coating of the predominant metallic contaminant present in the brine or water employed in the electrolytic process, usually Fe. Consequently, the Tafel slope characterizing the electrolytic activity of the applied coating changes to essentially that of iron, with a resulting increase in hydrogen overvoltage, especially at higher current densities, i.e. 1.5 to 3.5 amps per square inch and above, as are common in modern membrane chlor-alkali cells. In contrast, it is desirable to maintain the intrinsically low overvoltage properties of those materials which are known to be characterized by low Tafel slopes, i.e. platinum group metal oxides, particularly ruthenium oxide, during long-term operation in membrane chlor-alkali cells. It has now been discovered, among other things, that active coatings of oxides of platinum group metals and secondary electrocatalytic metals may be prepared which, when prepared according to the process of the invention, exhibit unexpected properties of low hydrogen overvoltage, physical stability, and long-term efficacy as cathodes in the electrolysis of brine at conditions of high NaOH concentrations, temperatures, and process pressures. It has also been discovered that the use of these electrodes in electrolytic processes wherein chlorine and caustic soda are produced at certain process conditions of temperature, NaOH concentration, pressure, etc., results in reduced energy requirements not otherwise attainable in practice.